Restoring stone monuments: Enlightening critical details by the combined use of innovative sensing techniques

The experience gathered during the implementation of long series of laboratory experimental protocols, aiming to study the mechanical response of restored structural elements, is summarized. Conclusions are drawn con­cerning the proper exploitation of the laboratory results in field applications in the frame of on-going restoration projects of ancient stone monuments. The need of continuous bidirectional interaction between the scientific per­son­­nel working in the site and the scientists working in the laboratory is empha­sized. The role of the Digital Image Correlation technique in quantifying para­sitic effects influencing the laboratory data is proven decisive. The need to use modern sensing techniques, providing data from the interior of loaded res­tored complexes (simulating restored structural el­ements), like the Pressure Stimu­lated Currents and the Acoustic Emissions ones, according to a com­bined manner is highlighted. The capability of these two techniques to monitor the damage evolution within the mass of the elements tested and to provide clear pre-failure indicators renders them flexible tools in the hands of engineers designing the restoration projects

Correlation between the electric and acoustic signals emitted during compression of brittle materials

An experimental protocol is described including a series of uniaxial compression tests of three brittle materials (marble, mortar and glass). The Acoustic Emission (AE) technique and the Pressure Stimulated Currents (PSC) one are used since the recordings of both techniques are strongly related to the formation of cracking in brittle materials. In the present paper, the correlation of these techniques is investigated, which is finally proven to be very satisfactory

The critical influence of some “tiny” geometrical details on the stress field in a Brazilian Disc with a central notch of finite width and length

The role of some geometrical characteristics of the notches ma­chined in circular discs, in order to determine the mode-I fracture tough­ness of brittle materials, is discussed. The study is implemented both analyti­cally and numerically. For the analytic study advantage is taken of a recently intro­duced solution for the stress- and displacement-fields developed in a finite disc with a central notch of finite width and length and rounded corners. The vari­ation of the stresses along strategic loci and the deformation of the peri­me­ter of the notch obtained analytically are used for the calibration/validation of a flexible nu­mer­ical model, which is then used for a parametric investiga­tion of the role of geometrical features of the notched disc (thickness of the disc, length and width of the notch, radius of the rounded corners of the notch). It is con­cluded that the role of the width of the notch is of critical im­port­ance. Both the ana­lytic and the numerical studies indicate definitely that ignoring the ac­curate geo­metric shape of the notch leads to erroneous results concerning the actual stress field around the crown of the notch. Therefore, it is possible that misleading values of the fracture toughness of the material of the disc may be obtained

A biomechanical study of the role of sitagliptin on the bone characteristics of diabetic rats

An experimental protocol is described aiming to explore the in­fluence of Type 2 Diabetes Mellitus on the biomechanical response of the bone tissue and, also, to quantify the potential beneficial role of a pharma­ceutical treatment, based on sitagliptin, a diabetes drug that increases the levels of natural substances called incretins. Twenty eight male, 10-week old Wistar rats were used, divided into three groups, i.e., the control one, the group including the diabetic rats and, finally, the group including the diabetic rats which were treated using sitagliptin. The biomechanical study was based on a series of three-point bending tests of the femora of the sacrificed rats and the analysis of the experimental data was implemented in terms of the actual geometry of the fractured cross-section. It was concluded that diabetic bones undertake larger forces despite the fact that the “diameter” of their cross-section was some­­how smaller. On the contrary, the slope of the load-deflection curve (cor­responding to a measure of the stiffness) of diabetic bones is slightly lower compared to the control bones. Finally, it seems that treating diabetic animals with sita­gliptin only partly reverses the effect of Type 2 Diabetes Mellitus on their bone tissue, at least concerning its strength and stiffness

The multi-layered ring under parabolic distribution of radial stresses combined with uniform internal and external pressure

A recently introduced solution for the stress- and displacementfields, developed in a multi-layered circular ring, composed of a finite number of linearly elastic concentric layers, subjected to a parabolic distribution of radial stresses, is here extended to encompass a more general loading scheme, closer to actual conditions. The loading scheme includes, besides the parabolic radial stresses, a combination of uniform pressures acting along the outer- and innermost boundaries of the layered ring. The analytic solution of the problem is achieved by adopting Savin’s pioneering approach for an infinite plate with a hole strengthened by rings. Taking advantage of the results provided by the analytic solution, a numerical model, simulating the configuration of a threelayered ring (quite commonly encountered in practical applications) is validated. The numerical model is then used for a parametric analysis enlightening some crucial aspects of the overall response of the ring

Analytical and numerical study of the stress field in a circular semi- ring under combined diametral compression and bending

The stress field developed in a circular semi-ring under the com­bined action of diametral compression and bending is explored both analytically and num­erically. The analytic solution is implemented by means of the complex potentials technique as it was formulated by Muskhe­lishvili, while for the nu­m­eri­cal study a finite element model, properly validated based on expe­ri­mental data, is used. The ana­lytic solution provided closed formulae for the stress field along strategic loci of the specimen, while the nu­m­eri­cal model permitted thor­ough parametric investigation of the de­pend­ence of critical quantities on geometrical and loading factors. The idea behind the study is to assess the potentialities of the circular semi-ring as a possible substitute of the familiar Brazilian disc, in the direction of curing drawbacks of the latter. It was concluded that a circular semi-ring subjected to eccentric diametral com­pression provides reliable data for the tensile strength of very brittle materials, relieved from ambiguities char­acterizing the standardized Brazilian-disc test

Acoustic Emissions versus Pressure Stimulated Currents during bending of restored marble epistyles: Preliminary results

The efficiency of two modern sensing techniques, namely the “Acoustic Emissions” and the “Pressure Stimulated Currents” ones, when they are used as Continuous Structural Health Monitoring tools, is assessed experimentally. The protocol includes multi-point bending of an accurate copy of a fractured marble epistyle of the Parthenon’s Temple on the Acropolis of Athens, under a scale of 1:3. The integrity of the epistyle is restored with three pairs of bolted titanium bars, according to the pioneering technique developed by the scientists of the “Committee for the Conservation of the Acropolis Monuments”. The data provided by the above techniques are considered in juxtaposition to each other and also in comparison to data provided by the “Digital Image Correlation” technique. It is concluded that, at least from a qualitative point of view, the data of all three techniques are in good mutual agreement. Combined exploitation of the various sets of experimental data enlightens interesting aspects concerning the succession of failure mechanisms activated during the loading procedure, revealing the critical role of the internal interfaces characterizing the restored epistyle. Moreover it is definitely indicated that both the “Acoustic Emissions” and the “Pressure Stimulated Currents” techniques provide clear signs of upcoming failure well before macroscopically visible damages are detected at the external surface of the specimen

Exploration of the optimum finite element modelling techniques for honeycomb structures for non-pneumatic tyre applications

Honeycomb spokes are among the most commonly used structures in non-pneumatic tyre (NPT) research and development. Even though finite element (FE) modelling plays a key role in this effort, there is still a lack of knowledge regarding the requirements for accurate FE simulation of the mechanical behaviour of NPT honeycomb structures. The use of an inappropriate FE type can lead to misleading results and act as a barrier for robust research and development. To address this gap in literature, the present study explores the optimum FE type for different relative dimensions of honeycomb NPT spokes. To this end, NPT segments with honeycomb spokes were 3D printed from TPU95 and subjected to compression tests. FE models were created using different element types (2D-plane quadrilaterals/triangles and 3D shell, higher order) to determine their accuracy when used in structures with varied spoke thickness-to-height ratios. 3D shells proved to be the optimum choice at predicting the required load for initial buckling for thickness-to-height ratios of 1/18 or smaller, and 2D plane elements (both quadrilaterals and triangles) proved optimum for ratios of 1/12 or larger

Correlation between the electric and acoustic signals emitted during compression of brittle materials

An experimental protocol is described including a series of uni¬axial compression tests of three brittle materials (marble, mortar and glass). The Acoustic Emission (AE) technique and the Pressure Stimulated Currents (PSC) one are used since the recordings of both techniques are strongly related to the formation of cracking in brittle materials. In the present paper, the correlation of these techniques is investigated, which is finally proven to be very satisfactory